Portland cement, blast furnace slag cement, and other cement are being widely used as soil reinforcers and as cement concrete for civil engineering structures or buildings.
As an advantage of cement, by pouring mortar or fresh concrete obtained by mixing a mixture of cement and aggregate with water into forms etc., it is possible to produce various shapes of structures and possible to produce concrete structures with high compressive strengths.
Further, cement can be produced by firing the limestone and clay present in large quantities and can be used mixed with blast furnace slag, fine ash, or other byproducts of other industries, so there is the advantage that it can be supplied inexpensively in large amounts.
Due to these advantages, cement is one of the most greatly used industrial products.
In cement, blast furnace slag cement is ground granulated blast furnace slag (GGBFS) obtained by finely grinding blast furnace slag (granulated blast furnace slag) having a high glassification rate alone or a mixture of ground granulated blast furnace slag and Portland cement etc.
Granulated blast furnace slag is a granular material containing a large amount of glass produced by water cooling blast furnace slag in a molten state at 1300 to 1500° C. so as to rapidly cool it. This granulated blast furnace slag is ground by a grinding mill to a specific surface area of 3000 cm2/g or more, in the case of a high activity product, 4000 to 6000 cm2/g. The ground granulated blast furnace slag is used as a cement material.
Note that, blast furnace slag is inorganic matter of numerous ingredients produced as a byproduct when producing pig iron at an ironmaking blast furnace. In general, it contains SiO2 in 30 to 35 mass %, CaO in 40 to 45 mass %, MgO in 2 to 8 mass %, and Al2O3 in 6 to 18 mass % and further contains, as trace ingredients, TiO2, CaS, FeO, etc.
If using granulated blast furnace slag with a glassification rate of 95% or more, good performance blast furnace slag cement can be produced.
If the water mixed in is alkali, the CaO and Al2O3 contained in the ground granulated blast furnace slag leach out from the powder into the water causing a hydration reaction contributing to the curing of the cement structure.
However, under conditions where the water is neutral or acidic, the ground granulated blast furnace slag is extremely slow in the setting reaction, so except for special cases, Portland cement and other mixed cement obtained by mixing strongly alkaline cement and ground granulated blast furnace slag is used.
In general, cement containing ground granulated blast furnace slag in a rate of 30 mass % or less has functions substantially equal to those of the cement mixed in. That is, the thus produced cured cement has an initial strength and final strength substantially equal to those of the cement mixed in. This mixed cement can be used for replacement of Portland cement in the building and civil engineering fields.
Further, cement containing ground granulated blast furnace slag in 30 to 70 mass % is slow in initial setting of the cured cement, but is high in final strength, is low in heat generation, and has other features, so is used for large-scale structures and civil engineering.
In this way, the ratio of mixture of the ground granulated blast furnace slag is changed in accordance with the application of the cement.
Further, blast furnace slag cement has a high seawater resistance and further an alkali aggregate reaction suppression effect etc. It is strong in durability in various adverse environments, so is used for wave-breaker blocks, bridge trestle concrete, etc.
Note that blast furnace slag includes low alumina slag (Al2O3:12% or less) and high alumina slag (Al2O3:12% or more).
High alumina ground granulated blast furnace slag releases a large amount of aluminum ions forming hydrates when the concrete cures. As a result, the concrete or mortar becomes higher in strength, so it is possible to produce good quality blast furnace slag cement from ground granulated blast furnace slag using high alumina blast furnace slag as a material.
In this way, blast furnace slag cement using high alumina ground granulated blast furnace slag has a high final strength as cured cement, so can be said to be superior in quality.
However, cement comprised of mainly high alumina ground granulated blast furnace slag and Portland cement sometimes expands over a long period from several years to 10 or more years after curing in soil containing sulfates due to the effect of the sulfate ions.
Aluminum ions are leached from the alumina in the blast furnace slag. Further, calcium ions are leached from the lime ingredients contained in the blast furnace slag and Portland cement. The calcium ions react with the sulfate ions, form sulfates, and finally form ettringite. This ettringite further reacts with the eluted aluminum ions and forms monosulfates of aluminum and calcium (monosulfate salts).
If after this concrete cures, sulfate ions further permeate it, the monosulfate salts and sulfate ions react whereby ettringite is again formed. At this time, the concrete increases in volume, so the cement concrete swells. In the worst cases, the concrete swells so much that the structure is destroyed.
Note that acid sulfate soil is mostly at regions in proximity to volcanoes and some coastal regions in Japan. Further, overseas, it is prevalent at dry regions such as the Middle East, the North American coast, etc. Among these soils, calcium sulfate, magnesium sulfate, sodium sulfate, etc. remain residually in the soil. Due to the effect of erosion of the cement concrete by the sulfates, the problem of degradation due to expansion of the cement concrete contacting the soil easily occurs.
To solve the problem of damage to the concrete structure by this sulfate expansion, Portland cement with low calcium aluminate resistant to sulfate expansion is mixed in with the blast furnace slag cement and the mixture used.
Further, in applications with particularly large effects of sulfates, sometimes the ratio of mixture of the high alumina ground granulated blast furnace slag is made 60 mass % or more, preferably 70 mass % or more.
The principle in the thinking in handling this is that since the Portland cement ratio falls and the leaching of calcium ions of the Portland cement is reduced, the balance of aluminum ions and calcium ions changes thereby creating a state in which the calcium ions required for formation of ettringite become insufficient and therefore the formation of ettringite can be prevented.
In mixed cement made of high alumina ground granulated blast furnace slag and Portland cement as main materials, various measures have been taken to prevent sulfate expansion.
For example, with the measure of using low alumina Portland cement and making the ratio of mixture of the ground granulated blast furnace slag 60 mass % or more, preferably 70 mass % or more, it was possible to suppress concrete expansion in a sulfate environment more than the case of Portland cement alone, but the initial setting of the concrete was slow. As a result, this cement could only be used for structures where slow initial setting is acceptable, that is, dams, embankments, and some other civil engineering applications. Therefore, with this measure, there was the problem that use for making concrete panels or tunnel segments and for concrete applications for building foundations was not possible.
Further, as the method for suppression of concrete expansion due to sulfates, for example, as described in Japanese Patent Publication (A) No. 8-12387, the practice has been to add sulfate ions reacting with the aluminum ions initially leached from the ground granulated blast furnace slag in advance in fresh concrete.
This method formed the ettringite at an early timing, that is, before the expression of the concrete strength, so as to reduce the formation of ettringite after curing of the concrete. Specifically, a large amount of plaster (CaSO4, sometimes anhydrous crystals and hydrous crystals) was added to the blast furnace slag cement to suppress the expansion under a sulfate environment.
However, in blast furnace slag cement including ground granulated blast furnace slag in an amount of 10 to 60 mass %, to raise the sulfate durability, even in the case of cement in which Portland cement with its greatest effect of suppression of sulfate expansion is mixed, it was necessary to making the amount of addition of plaster to the total amount of cement over 4 mass % converted to SO3.
However, the sulfate ions leached from the plaster further have the effect of delaying the cement setting, so if increasing the amount of addition of plaster, there was the problem that the extremely early setting of the concrete (within 1 to 3 days) became slow. As a result, applications requiring early setting, that is, applications to building foundations, concrete panels, tunnel segments, etc. were difficult.
Further, in concrete produced by cement in which a large amount of plaster is added, there was also the problem of a drop in the final strength. To suppress this effect, it was necessary to add plaster under conditions of 4 mass % or less converted to an SO3.
That is, in the prior art, there was never a method simultaneously solving the problems of the conditions of the concrete setting speed of mixed cement of high alumina ground granulated blast furnace slag and Portland cement and sulfate expansion.
Further, as a method of production of concrete with a high durability in an environment with many sulfate ions present and further in an acidic environment, for example as described in Japanese Patent Publication (A) No. 2005-35877 and Japanese Patent Publication (A) No. 2004-59396, it is described to add, in addition to cement, blast furnace slag powder with a 100 micron or less particle size, steelmaking slag powder, and slag aggregate with a glassification rate 10% or less.
However, this method improved the sulfuric acid resistance of concrete due to the mixing of materials at the time of laying the concrete. It did not improve the sulfate resistance performance of the cement itself. Therefore, in this method, application to locations where only general aggregate can be obtained and structures requiring use of high strength aggregate was difficult.
By using blast furnace slag as a raw material for blast furnace slag cement, it is possible to use the byproduct blast furnace slag produced in ferrous metal production as a high added value industrial raw material and possible to efficiently utilize resources and conserve on energy.
However, to expand this application, it was necessary to raise the durability of blast furnace slag cement using high alumina ground granulated blast furnace slag in an acid sulfate soil, but there has never been prior art satisfying the criteria of both this object and the cement setting performance.
Therefore, to overcome these defects in the prior art, technology for production of blast furnace slag cement having both a high sulfate resistance performance and a setting performance equal to that of conventional cement has been sought.